In general, a fuel cell system is a power generation system which generates electric energy by an electrochemical reaction of hydrogen and oxygen or oxygen in air in a fuel cell. For example, the fuel cell system has been used in a fuel cell vehicle to operate an electric motor and drive the vehicle. The fuel cell system includes a stack, where unit fuel cells including a cathode and an anode generate electricity, an air supplying device supplies air to the cathode of the fuel cell, and a hydrogen supplying device supplies hydrogen to the anode of the fuel cell.
Meanwhile, in a polymer fuel cell, substantial amount of moisture is required for functioning of an ion-exchange membrane in a membrane-electrode assembly (MEA), and thus, the air supplying device of the fuel cell system includes a humidifier for humidifying air which is supplied to the fuel cell. The humidifier supplies moisture in air at an elevated temperature or moisture discharged from the cathode of the fuel cell to dry air which is supplied through an air compressor of the air supplying device, and then, supplies the humidified air to the cathode of the fuel cell.
Examples of the humidifier may include various types such as a bubbler type, an injection type, a plate type, an absorbent type, a membrane type, and the like. However, since the fuel cell vehicle has a limitation in a package space, the membrane-type humidifier having a reduced volume has been used in the fuel cell vehicle. In particular, the membrane-type humidifier may provide advantages in packing and particular power source may not be required for the membrane-type humidifier.
The membrane-type humidifier, which is, hereafter referred to “membrane humidifier”, may use membrane for humidifying by exchanging moisture from a gas to gas such as between discharged gas at the elevated temperature and humidity from the cathode of the fuel cell and dry air supplied through the air compressor.
The humidified air generated by the membrane humidifier is supplied to the cathode of the fuel cell and the discharged gas from which moisture is removed in the membrane humidifier is supplied into the air. For example, the membrane humidifier includes a membrane module in which hollow fiber membranes are substantially disposed within a housing having a cylindrical shape and manifolds of shell type which are formed at both sides of the membrane module.
Meanwhile, according to the related art, a structure of the humidifier has been provided and a portion of the dry air which is supplied through the air compressor may be sprayed into a discharge path of the discharged gas to dilute hydrogen in the discharged gas from which moisture is removed from the humidifier into the air. Since the hydrogen is contained in the discharged gas into the air in the fuel cell vehicle, hydrogen in the discharged gas is required to be diluted as described above according to environmental and safety regulation for discharging gases.
Accordingly, in the related art, a bypass line which is branched from an air supplying line that connects the air compressor and the humidifier to and connected to a discharge line of the discharged gas of the humidifier has been provided, and an open and close valve for selectively opening or closing a channel of the bypass line has been provided. However, as a separate bypass line connecting the air supplying line and the discharge line of the discharged gas to each other is formed, a space for installing the bypass line may be required, and as such, efficient package may not be obtained.
Although the membrane humidifier may occupy less volume than other types of humidifiers, scheme for exchanging the gas with gas moisture may still require a substantial volume as a part in a vehicle. Particularly, the additionally used bypass line as described above may not be efficient.
Further, according to the related art, water condensate may be generated in the membrane humidifier as the membrane humidifier operates. Since such water condensate may be frozen in cold weather condition, an air channel in the humidifier may be reduced thereby causing increase the burden of the air compressor as the pressure rises, and a power consumption of the air compressor may increase, thereby deteriorating fuel efficiency of the fuel cell vehicle.
In addition, when the water condensate in the humidifier is frozen in cold weather condition, the membrane module of the humidifier may be damaged and humidifying performance of the humidifier may be deteriorated due to the volume of the water condensate increases by forming ice.
The above information disclosed in this Background section is merely for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.